Question

A simple expression:

Object val = true ? 1l : 0.5;

What type is val? Well, logically, val should be a Long object with value 1. But Java thinks that val is a Double with value 1.0.

It doesn’t have to to anything with autoboxing as

Object val = true ? new Long(1) : new Double(0.5);

results with same behavior.

Just to clarify:

Object val = true ? "1" : 0.5;

results in the correct String.

Can anybody explain me why they have defined this like that? For me it seems to be rather really bad designed or actually a bug.

Answer 1

This is described in the Java Language Specification Section 15.25 (relevant parts in bold face):

The type of a conditional expression is determined as follows:

1. If the second and third operands have the same type […]

2. If one of the second and third operands is of type boolean […]

3. If one of the second and third operands is of the null type […]

4. Otherwise, if the second and third operands have types that are convertible (§5.1.8) to numeric types, then there are several cases:

   1. If one of the operands is of type byte […]

   2. If one of the operands is of type T where T is byte, short, or char, […]

   3. If one of the operands is of type Byte […]

   4. If one of the operands is of type Short […]

   5. If one of the operands is of type; Character […]

   6. Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands. Note that binary numeric promotion performs unboxing conversion (§5.1.8) and value set conversion (§5.1.13).

5. Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7).

The “lub” mentioned in the last paragraph stands for least upper bound and refers to the most specific super type common for T1 and T2.

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As for the case with Object val = true ? 1l : 0.5; I agree that it would be more precise if it applied rule 5 (on the boxed values). I guess the rules would become ambiguous (or even more complicated) when taking autoboxing into account. What type would for instance the expression b ? new Byte(0) : 0.5 have?

You can however force it to use rule 5 by doing

Object val = false ? (Number) 1L : .5;